Page 124 - DMGT505_MANAGEMENT_INFORMATION_SYSTEM

Unit 6: Business Networks and Telecommunications

A default gateway is a router that a host is configured to trust for routing traffic to remote Notes
systems across the network. However, the trusted router must be attached to the same network
as the trusting host. A router on a remote network cannot be used for providing the functionality
of the default gateway.

ICMP

The Internet Control Message Protocol (ICMP), an error reporting protocol that is an integral
part of the IP protocol. ICMP communicate control data, information data, and error recovery
data across the network. Problems that is less severe than transmission errors result in error
conditions that can be reported.

Example: Suppose some of the physical paths in Internet fail, causing the Internet to be
partitioned into two sets of networks with no path between the sets, a datagram sent from a host
in one set to a host in other cannot be delivered.
The TCP/IP suite includes a protocol called ICMP that IP uses to send error messages when
condition such as the one described above arises. The protocol is required for a standard
implementation of IP. We will see that the two protocols are co-dependent. IP uses ICMP when
it sends an error message, and ICMP uses IP to transport messages. Following is a brief description
of some of the error messages defined by ICMP protocol:
1. Source Quench: A router or host whose receive communication buffers are nearly full
normally triggers this message. A source quench message is sent to the sending host, the
receiver is simply requesting the sending host to reduce the rate at which it is transmitting
until advised otherwise.

2. Time Exceeded: A time-exceeded message is sent in two cases. Whenever a router reduces
the TTL field in a datagram to zero, the router discards the datagram and sends a time-
exceeded message. In addition, a time-exceeded message is sent by a host if the reassembly
timer expires before all fragments from a given datagram arrive.

3. Route Redirect: A router sends this message to a host that is requesting its routing services.
When a host creates a datagram destined for a network, it sends the datagram to a router,
which forwards the datagram to its destination. If a router determines that a host has
incorrectly sent a datagram that should be sent to a different router, the router uses route
redirect message to cause the host to change its route. In this manner, a route redirect
message improves the efficiency of the routing process by informing the requesting host
of a shorter path to the desired destination.

4. Host unreachable: Whenever a gateway or a router determines that a datagram cannot be
delivered to its final destination (due to link failure or bandwidth congestion), an ICMP
host unreachable message is sent to the originating node on the network. Normally, the
message includes the reason the host cannot be reached.

5. Fragmentation and Reassembly: The largest datagram the IP protocol can handle is 64
Kbytes. The maximum datagram size is dictated by the width of the total length field in
the IP header. Realistically, most underlying data link technologies cannot accommodate
this data size.

Example: The maximum size of the data frame supported by Ethernet is 1,514 bytes.
Unless rectified, something is done about situations like this. IP has to discard data that is delivered
to it from upper-layer protocols with sizes exceeding the maximum tolerable size by the data link
layer. To circumvent this difficulty, IP is built to provide data fragmentation and reassembly.

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